The present invention is an measurement system; and, more particularly, to a near-field optical heterodyne measurement system using a near-field fiber-optic probe, by using which characteristics of high-speed and high-frequency devices such as HBT and PHEMT can be measured with a high resolution and a low noise.
Near-field fiber-optic probes have attracted a great deal of current interest mainly due to their ability to extend optical microscopy beyond the classical diffraction limit. Scanning optical microscopes using these probes have yielded very high resolution in imaging and spectroscopic applications at visible and infrared wavelengths. The technology has been applied to imaging in biology material science, surface chemistry and information storage.
In conventional research on an optical heterodyne technique, micro- and millimeter waves can be generated by the injection of a laser beam into a high-speed and high-frequency semiconductor element through a lens or a single mode optical fiber. In this case, however, a diameter of the laser beam is much larger than that of the semiconductor element, thereby degrading a stability of a detected signal and resulting in a high noise figure.
It is, therefore, an object of the present invention to provide a near-field optical heterodyne measurement system using a near-field fiber-optic probe, by which characteristics of high-speed and high-frequency devices such as HBT and PHEMT can be measured with a high resolution in imaging and a low noise.
In accordance with an aspect of the present invention, there is provided a near-field optical heterodyne measurement system for measuring characteristic of high-frequency and high-speed devices, comprising: a combining means for combining two optical beams to produce a submicron-size optical beam, wherein the two optical beams have different frequency from each other; a near-field fiber-optic probe for injecting the submicron-size optical beam into a sample device to be measured; a position controlling means for controlling a position of the near-field fiber-optic probe; and a analyzing means for receiving a millimeter wave exited from the sample device to measure characteristics of the sample device.
The combining means includes: a first laser for producing a first optical beam; a second laser for producing a second optical beam; and a fiber coupler for combining the first optical beam with the second optical beam.
The near-field fiber-optic probe includes: a waveguide for transmitting the combined optical beam from the fiber coupler; a tip, whose diameter is less than that of a wavelength of incident light; and a taper region for connecting the waveguide with the tip.
The position controlling means includes: a microscope objective for controlling a position of the near-field fiber-optic probe and the sample device; a feedback controller for setting a reference position to control a distance between the near-field fiber-optic probe and the sample device to be measured; a positioning controller for placing the near-field fiber-optic probe within a predetermined distance from a surface of the sample device to be measured; a function generator for setting a reference frequency and controlling a resonance frequency; a lock-in amplifier for measuring AC signal which is generated when the distance between the near-field fiber-optic probe and the sample device is changed; a fine vibrator, coupled to the near-field fiber-optic probe, for vibrating the near-field fiber-optic probe; a helium-neon laser for generating a laser light to control the distance between the near-field fiber-optic fiber and the sample device; and a photodiode for sensing the laser light generated from the helium-neon laser to control a distance between the near-field fiber-optic probe and the device to be measured.
Further, the measuring means includes: a fine probe for detecting the millimeter wave exited from the sample device to be measured; a local oscillator for producing a reference frequency; a mixer for mixing the reference frequency from the local oscillator with the millimeter wave to generate an intermediate frequency; a spectrum analyzer for measuring the intermediate frequency; and a parameter analyzer for analyzing parameter including a DC photocurrent to measuring device characteristics.